Because of their relatively complicated geometry, most dual burner swirlers require complicated and expensive manufacture. Particularly in premixing burners, no separation zones--due to the swirler included there as a constituent part in dual burners--must occur because such separation zones greatly increase the danger of burn-back.
A known way of constructing a separation-free swirler consists in providing a tubular shell with tangential inlet slots. This produces a free vortex which flows away axially. It is found, however, that vortex reverse flow zones (vortex breakdown) have very poor stability properties in a free vortex.